Vacuum-assisted gravity casting apparatus using a porous, gas permeable casting mold confronted by a vacuum box are described in such prior art patents as the Wilkins U.S. Pat. No. 2,908,054; the Burke U.S. Pat. No. 3,608,617; and the Miura U.S. Pat. No. 3,825,058. The gas permeable mold typically includes an upper sprue exposed outside the vacuum box and through which the melt is gravity poured into the mold to fill the mold cavity therein. The vacuum box confronts the mold in a manner to permit evacuation of the mold cavity through the mold walls to not only remove gas (e.g., air) from the mold cavity that might otherwise be trapped in the casting produced therein but also to facilitate rapid, complete filling of the mold cavity with the melt. The relative vacuum is usually established in the mold cavity prior to and during filling of the mold cavity with the melt and may be discontinued once the mold cavity is completely filled.
Vacuum-assisted, countergravity casting apparatus using a porous, gas permeable casting mold confronted by a vacuum box are described in such prior art patents as the Chandley et al U.S. Pat. Nos. 4,340,108 and 4,606,396. Typically, the countergravity casting apparatus includes a mold having a porous, gas permeable upper cope and a lower drag sealingly secured together at a parting plane, a vacuum box confronting the gas permeable cope, and means for immersing (exposing) the underside of the drag in an underlying pool of melt. The vacuum box is evacuated sufficiently by a suitable vacuum pump while the drag underside is immersed in the pool to draw the melt upwardly through one or more ingate passages in the drag into one or more mold cavities formed between the cope and drag.
Typically, this countergravity casting process fills the mold cavities with the melt in a relatively short time, such as approximately 2-3 seconds. However, the mold must remain immersed in the melt until at least the melt in the ingate passages of the mold drag freezes off (solidifies). For example, the mold typically must remain immersed in the melt pool on the order of 15 to 50 seconds after filling of the mold cavity to freeze off (solidify) the melt in the ingate passages. In the event the mold is withdrawn from the melt pool prior to freezing of the melt in the ingate passages, the melt in the ingate passages as well as in the mold cavities can flow downwardly out of the mold and result in a defective casting which must be scrapped. The time required to freeze off the melt ingate passages has thus limited the rate of production of castings using this process.
In attempts at reducing the casting cycle time, prior art workers have tried disengaging the mold from the melt before the melt in the ingate passages is solidified while providing an increased vacuum level in the vacuum box sufficient to prevent the unsolidified melt from draining out of the ingate passages and mold cavities. However, as the level of the relative vacuum in the vacuum box is increased to this end, penetration of the porous mold by the melt (known as "burn-in") in upper mold regions proximate the vacuum source has been observed and resulted in production of defective castings which must be scrapped. "Burn-in" has been attributed to excessively high vacuum levels at upper mold regions as a result of their close proximity to the vacuum source; i.e., as a result of an increasing vacuum level gradient from the lower mold regions remote from the vacuum source to the upper mold regions proximate the vacuum source. Thus, difficulty has been experienced in providing a vacuum level at the drag ingate passages strong enough to hold the unsolidified melt therein during mold/melt disengagement without at the same time causing "burn-in" at other upper mold regions.
It is an object of the invention to provide an improved apparatus for the vacuum-assisted casting of a melt into a gas permeable mold having a first mold portion confronted by a vacuum box and a second mold portion unconfronted by the vacuum box wherein a substantially ga impermeable seal is cooperatively located relative to the second mold portion to inhibit ambient gas flow therethrough during casting and thereby improve the uniformity of the vacuum level achievable about the mold and wherein a refractory shield is cooperatively located relative to the seal for protecting it from heat from an underlying source of the melt during casting.
It is another object of the invention to provide an improved apparatus for the vacuum-assisted, gravity casting of a melt into a gas permeable mold wherein a substantially gas impermeable seal and associated protective refractory shield are cooperatively disposed relative to a mold portion unconfronted by a vacuum box so as to provide a more uniform vacuum level about the mold during filling of the mold with the melt.
It is still another object of the invention to provide an improved apparatus for the vacuum-assisted, countergravity casting of a melt into a gas permeable mold wherein a substantially gas impermeable seal and associated protective refractory shield are cooperatively disposed relative to a lower mold portion unconfronted by a vacuum box so as to achieve a strong enough vacuum level at the lower mold ingate passages to hold unsolidified melt therein to permit early mold/melt disengagement before melt solidification, and thus reduced casting cycle times, without at the same time experiencing "burn-in" at other upper mold regions.